In the determination of an object shape from digitally recorded holograms, Fourier holography, also in multi-wavelength technology, constitutes a good approach to be able to perform the digital reconstruction with comparatively simple algorithms. This was illustrated, among others, in the scientific paper “Digital recording and numerical reconstruction of lensless Fourier-Hologramms in optical metrology” by the authors Christoph Wagner, Sönke Seebacher, Wolfgang Osten, and Werner Jüptner in Applied Optics of Aug. 1, 1999, vol. 38, no. 22, pages 4812 to 4820 [1]. The paper clearly states that Fourier holography can be implemented comparatively well as a holographic method with digital recording, since when the geometry parameters of the holographic arrangement are selected accordingly, the interference fringe densities occurring in the hologram can technically be controlled comparatively well by means of existing digital cameras.
In addition, short-coherence holography constitutes an absolute-measurement measuring method, the potential and advantages of which have been clearly recognized already several years ago, cf. the scientific paper “Applications of short-coherence digital holography in microscopy” by Lluis Martinez-León, Giancarlo Pedrini, and Wolfgang Osten in Applied Optics of Jul. 1, 2005, vol, 44, no. 19, pages 3977 to 3984 [2]. This paper particularly discusses the possibilities and limitations for obtaining a high lateral resolution—as in [1] as well—when determining an object form, while [2] also emphasizes the possibility of depth discrimination particularly for biological objects.
One application of short-coherence holography on the basis of the Fourier approach is shown in the scientific paper [3] “Lensless digital holography with short coherence light source for three-dimensional surface contouring of reflecting micro objects” by Caojin Yuan, Hongchen Zhai, Xiaolei Wang, and Lan Wu in Optics Communications 270 (2007) 176-179. This paper gives an account of a method for digital short-coherence holography according to the Fourier approach for reflecting micro objects with a mechanical depth scan of the object. Here as well, no lenses are used for object imaging and this method also allows optical sectioning, as illustrated in [2]. Here, it is reported that by means of the short-coherence approach particularly the influence of speckling on the reflecting rough object can be reduced. In addition, [3] convincingly illustrates the potential of short-coherence holography in the measurement of rough object surfaces with strong inclination, here for example conical depressions with a large aspect ratio.
As described in [3], in the method for short-coherence holography, only one single area, which—depending on the coherence length of the light source used—is comparatively small in its depth extension, can be addressed holographically. The depth extension of the area is determined by the coherence length of the light used, so that an object with large depth extension and reduced coherence length can only be detected in a comparatively lengthy mechanical object scan with capturing of a plurality of holograms. Thus, when a short-coherent source is applied, it is only possible to holographically detect the object area which in the capturing process in the plane of the hologram detection has an optical path difference smaller than the coherence length of the light forming the hologram.
In a model-like image, this means for the approach according to [3] that an object is scanned in depth with a rung ladder, which only includes one ladder rung and which is gradually shifted in depth to scan the object. In each depth position of the ladder rung, one or more holograms are detected from one or more object points of the object. Therefore, this short-coherence holography method described in [3] can be time-consuming and is thus limited to rather small objects.
Moreover, objects that are further remote, for example at a distance of a few meters, cannot be detected by means of short-coherence holography in the prior art as illustrated in [2] and [3].